This invention relates to a control system for controlling the energization of electrically operated multizone heat generating apparatus, and in particular, to a control system for such apparatus having at least two parallel connected heat generating stages provided in each of the zones served by the apparatus.
In recent years, the use of a single electrically operated heat generating apparatus to provide warm air to a plurality of zones in a common enclosure has become quite popular. Buildings such as small office buildings and shopping malls, have proven to be particularly suitable for such apparatus whereat the heat generating apparatus may be conveniently located on the roof of the building. Generally, installations using such multizone heat generating apparatus are commercial in nature. The users of electrical power are charged by the power company at commercial rates which are generally based upon the total watt usage multiplied by a number indicative of the demand rate. The demand rate multiplier is typically based on the peak 30 minutes of usage during any billing period.
During the summer and winter seasons, the daily usage of the electric heat generating apparatus is generally constant. However, during the intermediate seasons of spring and fall, usage rates may fluctuate quite substantially. In particular, the temperature of the ambient may be at a relatively low level during the evening hours; whereas, during the daylight hours only a minimal quantity of heat may be required to maintain a comfortable level within the various zones of the building. At such time, during the evening hours, the building may be unoccupied whereby the thermostatic control governing operation of the heat generating apparatus may be reset to commence operation at a relatively low temperature level. However, prior to the building being reoccupied the following day, the thermostats are reset to their normal operating temperature levels and the heat generating apparatus is required to rapidly raise the temperature of the various zones to a normal level. Accordingly, the peak demand generally occurs during this warm-up period when the temperature level is raised from a relatively low level established during the unoccupied hours to the relatively warm level required to maintain the comfort of the occupants during the normal occupied hours.
Generally, at least two stages of heating are provided in each of the zones. Typically, a building is zoned so that a portion of the zones include a peripheral building wall (peripheral zones) and a different portion of the zones are located in interior sections of the building. Obviously, the warmup period will be of less duration for the interior zones than for the peripheral zones which have greater transmission losses to the ambient.
It has been suggested that one way to minimize the peak demand rate during the intermediate seasons is to block out all but the first stage of heating in each of the zones. While the foregoing would minimize the peak load on the heat generating apparatus, the warmup period would be relatively slow, particularly for the peripheral zones. Accordingly, the suggested solution to the problem is not satisfactory from a user's standpoint.